The present invention relates in general to the pouring of molten metal into molds for manufacturing cast metal articles, and, more specifically, to stopper-controlled pouring of metal from a tiltable vessel wherein the molten metal flow is accurately directed into a sprue cup of the mold.
One type of automated pouring device for filling casting molds with molten metal includes a stopper-controlled pouring vessel. One example of such a pouring vessel utilizing a coreless induction heater is shown in U.S. Pat. No. 5,282,608. There is an inlet for admitting molten metal into a main holding chamber within the vessel and a bottom nozzle outlet for discharging the metal into underlying casting molds. A mechanically operated stopper rod interacts with the nozzle to regulate the flow of molten metal through the nozzle.
In order to optimize the properties of the cast article, a variable flow rate into the mold is necessary. Initially during the pouring of a mold, a high rate of metal flow from the pouring vessel into the mold is desired. Metal is poured into a sprue cup formed in the top of the mold and drains from the sprue cup through passages into the mold cavity. The sprue cup must be quickly filled to provide a smooth and even flow of metal into the mold cavity. Once the level of molten metal in the sprue cup reaches the desired height, a slower rate is maintained that matches the flow of metal out of the sprue cup into the mold cavity. This rate is maintained until sufficient metal has been poured to fill the mold cavity. Preferably, the flow of metal is stopped in time to avoid overspill of metal outside the sprue cup after the mold cavity is filled.
In a conventional stopper-controlled pouring system, a variable rate of molten metal flow through the nozzle is obtained by controlling the stopper rod height over the nozzle. Specifically, the rate of flow is given byR=δAk√{square root over (2gh)}where R is the rate of flow in pounds/second, A is the area of the orifice between the stopper rod and the nozzle in square inches, δ is the molten metal density in pounds per cubic inch, g is the gravitational constant, h is the head height of the molten metal bath above the orifice, and k is a constant which is the product of a coefficient of velocity, a coefficient of turbulence, and a coefficient of viscosity.
In prior art stopper-controlled pouring systems, the variable A is controlled in order to achieve a desired profile of the flow rate during mold filling. The above equation is solved for A and a controller uses a known target flow rate at any moment together with nominal constant values for δ and k in order to determine the appropriate stopper rod position corresponding to the solved value for area A. The value of A is approximate since during a particular pour, certain elements of the equation are in fact not constant. In particular, the height of the metal bath h changes as the metal in the chamber is consumed and the coefficients of velocity and turbulence may change as a result of the change in h.
It is possible to measure these changing values so that they can be updated dynamically within the controller during the pour and used to update the above equation. However, this adds complication and expense to the pouring system and may still yield unsatisfactory results. Area A and flow rate R are directly related so that a robust control is achieved. Flow rate R varies exponentially as a function of head height h, making control of flow rate R more difficult.
A target flow rate in a typical casting application may range from about 3 lbs/sec to about 30 lbs/sec, for example. A maximum depth of the metal bath may be about 24 inches. In order to accommodate the ability to pour at 30 lbs/sec when the bath height is depleted down to 4 inches, a relatively large nozzle diameter is required in order to achieve the necessary area A. When pouring at the slower rate of 3 lbs/sec when the height of the metal bath is 24 inches, the stopper height over the nozzle necessary to achieve the desired value for area A is very small due to the large nozzle diameter. Under these conditions, the change in flow rate is very sensitive to minute changes in the stopper position. Consequently, the flow rate is hard to control and becomes inconsistent from pour to pour because of the variable head height. A further problem is that, at small stopper heights, the metal flow through the nozzle begins to roostertail due to an increased velocity.
Previous attempts have been made in stopper-controlled pouring systems to maintain a constant head height in the molten metal bath. However, these attempts have been impractical and required complicated and expensive apparatus. For example, pressurized displacement of molten metal from a main chamber into a pouring subchamber has been used to provide a constant head height. In addition to the added expense, such a system required frequent maintenance resulting in down time and loss of productivity.
In order to increase productivity, it is desirable to pour metal into molds as the molds are carried in a conveyor line without stopping as is described in U.S. Pat. No. 5,056,584. As shown in that patent, a pouring vessel is suspended by a moving carriage in order to synchronize its movement with the moving molds. In a moving system, the pouring unit must have good mobility and should be contained completely above the height of the top of the moving molds on their conveyor system. The weight, complexity, and space requirements of prior art pouring systems having constant head height, however, have been unsuitable for these moving applications.
In related application Ser. No. 10/255,306, a stopper-poured vessel is tilted to an appropriate position during pouring such that a constant head height is obtained. The depth of the vessel is greater at the end opposite from the end where the stopper nozzle is located. The height of molten metal at the nozzle is measured and the tilt angle of the vessel is adjusted so as to maintain the desired height.
The molten metal flow discharged from the nozzle is directed into a sprue cup which channels the molten metal into the mold chambers. It is an objective in most metal foundry operations to minimize the size of a sprue cup in order to maximize efficiency. “Casting yield” is measured as a percentage of the gross casting mold weight divided by the net casting weight. For example, if the gross mold weight of a sand mold is 150 pounds and the net weight of the cast article is 85 pounds, then casting yield is 56.7%. A larger diameter sprue cup requires an larger depth of the sprue cup since the sides of the sprue cup must maintain an optimum slope (e.g., the sides are typically cone-shaped or rounded). Since the mold height must be increased to obtain a larger diameter sprue cup, the gross weight of the mold is also increased and the casting yield is decreased. A smaller sprue cup thus improves the casting yield.
In a tilting vessel, the nozzle exit is kept directly over the sprue cup by orienting the tilt axis through the nozzle exit; but the nozzle angle also tilts. The resulting flow of molten metal out of the nozzle has a corresponding trajectory with a small sideways component. This trajectory tends to require a larger sprue cup in order to ensure that the molten metal flow always hits the sprue cup.